Method and apparatus for transmitting a D2D signal by applying offset in wireless communication system

ABSTRACT

An embodiment of the present invention provides a device-to-device (D2D) signal transmission method wherein a terminal acquires synchronization and transmits a D2D signal by applying an offset in a wireless communication system, the method comprising the steps of: acquiring synchronization from at least one of a global navigation satellite system (GNSS) and an eNB; and transmitting a D2D signal on the basis of the acquired synchronization, wherein, when the terminal a) is located within a time division duplex (TDD) cell and b) is an in-coverage UE, and c) the eNB configures a system frame number (SFN) boundary on the basis of GNSS timing, the terminal applies a predetermined offset when a D2D signal is transmitted, regardless of whether the GNSS is used as a synchronization reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage application under 35 U.S.C. § 371of International Application No. PCT/KR2017/012254, filed on Nov. 1,2017, which claims the benefit of U.S. Provisional Application No.62/422,021, filed on Nov. 14, 2016, and U.S. Provisional Application No.62/416,140, filed on Nov. 1, 2016. The disclosures of the priorapplications are incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a wireless communication system, andmore particularly, to a method and apparatus for transmitting adevice-to-device (D2D) signal by applying an offset to the D2D signal,when a global navigation satellite system (GNSS) is available as asynchronization reference.

BACKGROUND ART

Wireless communication systems have been widely deployed to providevarious types of communication services such as voice or data. Ingeneral, a wireless communication system is a multiple access systemthat supports communication of multiple users by sharing availablesystem resources (a bandwidth, transmission power, etc.) among them. Forexample, multiple access systems include a code division multiple access(CDMA) system, a frequency division multiple access (FDMA) system, atime division multiple access (TDMA) system, an orthogonal frequencydivision multiple access (OFDMA) system, a single carrier frequencydivision multiple access (SC-FDMA) system, and a multi-carrier frequencydivision multiple access (MC-FDMA) system.

Device-to-device (D2D) communication is a communication scheme in whicha direct link is established between user equipments (UEs) and the UEsexchange voice and data directly without intervention of an evolved NodeB (eNB). D2D communication may cover UE-to-UE communication andpeer-to-peer communication. In addition, D2D communication may beapplied to machine-to-machine (M2M) communication and machine typecommunication (MTC).

D2D communication is under consideration as a solution to the overheadof an eNB caused by rapidly increasing data traffic. For example, sincedevices exchange data directly with each other without intervention ofan eNB by D2D communication, compared to legacy wireless communication,network overhead may be reduced. Further, it is expected that theintroduction of D2D communication will reduce procedures of an eNB,reduce the power consumption of devices participating in D2Dcommunication, increase data transmission rates, increase theaccommodation capability of a network, distribute load, and extend cellcoverage.

At present, vehicle-to-everything (V2X) communication in conjunctionwith D2D communication is under consideration. In concept, V2Xcommunication covers vehicle-to-vehicle (V2V) communication,vehicle-to-pedestrian (V2P) communication for communication between avehicle and a different kind of terminal, and vehicle-to-infrastructure(V2I) communication for communication between a vehicle and a roadsideunit (RSU).

DISCLOSURE Technical Problem

An aspect of the present disclosure is to provide a method oftransmitting a device-to-device (D2D) signal by applying an offset tothe D2D signal, when a global navigation satellite system (GNSS) isavailable as a synchronization reference.

It will be appreciated by persons skilled in the art that the objectsthat could be achieved with the present disclosure are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present disclosure could achieve will be more clearlyunderstood from the following detailed description.

Technical Solution

In an aspect of the present disclosure, a method of acquiringsynchronization and transmitting a device-to-device (D2D) signal by auser equipment (UE) in a wireless communication system includesacquiring synchronization from at least one of a global navigationsatellite system (GNSS) or an evolved Node B (eNB), and transmitting aD2D signal based on the acquired synchronization. When the UE is a)located within a time division duplex (TDD) cell and b) an in-coverageUE, and c) the eNB configures a system frame number (SFN) boundary basedon a GNSS timing, the UE applies a predetermined offset to transmissionof a D2D signal irrespective of whether the UE uses the GNSS as asynchronization reference.

In another aspect of the present disclosure, a UE for acquiringsynchronization and transmitting a D2D signal in a wirelesscommunication system includes a transmitter, a receiver, and aprocessor. The processor is configured to acquire synchronization fromat least one of a GNSS or an eNB, and to transmit a D2D signal based onthe acquired synchronization through the transmitter. When the UE is a)located within a TDD cell and b) an in-coverage UE, and c) the eNBconfigures an SFN boundary based on a GNSS timing, the processor isconfigured to apply a predetermined offset to transmission of a D2Dsignal irrespective of whether the UE uses the GNSS as a synchronizationreference.

The D2D signal may include information indicating whether thepredetermined offset is applied in a physical sidelink broadcast channel(PSBCH).

Upon receipt of the PSBCH, an out-coverage UE may apply thepredetermined offset to transmission of a D2D signal.

When the UE is a) located within the TDD cell and b) an in-coverage UE,c) the eNB does not configure the SFN boundary based on the GNSS timing,and d) the UE uses the GNSS as a synchronization reference, the UE mayapply a D2D frame number (DFN) offset to transmission of a D2D signal.

The DFN offset may be obtained by applying the predetermined offset to adifference between an SFN and a DFN.

The DFN offset may be equal to an offset preconfigured for anout-coverage UE.

Even though a specific subframe partially overlaps with a subframe of aUE within the TDD cell, the UE may not exclude the specific subframefrom a D2D subframe.

Advantageous Effects

According to the present disclosure, ambiguity in applying an offsetbetween user equipments (UEs) and interference caused by the resultingtiming difference may be overcome in a situation in which differentsynchronization references co-exist.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present disclosure are not limited to whathas been particularly described hereinabove and other advantages of thepresent disclosure will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this application, illustrate embodiments of the disclosure andtogether with the description serve to explain the principle of thedisclosure. In the drawings:

FIG. 1 is a view illustrating the structure of a radio frame;

FIG. 2 is a view illustrating a resource grid during the duration of onedownlink slot;

FIG. 3 is a view illustrating the structure of a downlink subframe;

FIG. 4 is a view illustrating the structure of an uplink subframe;

FIG. 5 is a view illustrating the configuration of a wirelesscommunication system having multiple antennas;

FIG. 6 is a view illustrating a subframe carrying a device-to-device(D2D) synchronization signal;

FIG. 7 is a view illustrating relay of a D2D signal;

FIG. 8 is a view illustrating an exemplary D2D resource pool for D2D;

FIG. 9 is a view illustrating a scheduling assignment (SA) period;

FIG. 10 is a view illustrating an embodiment of the present disclosure;and

FIG. 11 is a block diagram of transmission and reception apparatuses.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present disclosure described hereinbelow arecombinations of elements and features of the present disclosure. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent disclosure may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent disclosure may be rearranged. Some constructions or features ofany one embodiment may be included in another embodiment and may bereplaced with corresponding constructions or features of anotherembodiment.

In the embodiments of the present disclosure, a description is made,centering on a data transmission and reception relationship between abase station (BS) and a user equipment (UE). The BS is a terminal nodeof a network, which communicates directly with a UE. In some cases, aspecific operation described as performed by the BS may be performed byan upper node of the BS.

Namely, it is apparent that, in a network comprised of a plurality ofnetwork nodes including a BS, various operations performed forcommunication with a UE may be performed by the BS or network nodesother than the BS. The term ‘BS’ may be replaced with the term ‘fixedstation’, ‘Node B’, ‘evolved Node B (eNode B or eNB)’, ‘Access Point(AP)’, etc. The term ‘relay’ may be replaced with the term ‘relay node(RN)’ or ‘relay station (RS)’. The term ‘terminal’ may be replaced withthe term ‘UE’, ‘mobile station (MS)’, ‘mobile subscriber station (MSS)’,‘subscriber station (SS)’, etc.

The term “cell”, as used herein, may be applied to transmission andreception points such as a base station (eNB), a sector, a remote radiohead (RRH), and a relay, and may also be extensively used by a specifictransmission/reception point to distinguish between component carriers.

Specific terms used for the embodiments of the present disclosure areprovided to help the understanding of the present disclosure. Thesespecific terms may be replaced with other terms within the scope andspirit of the present disclosure.

In some cases, to prevent the concept of the present disclosure frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

The embodiments of the present disclosure can be supported by standarddocuments disclosed for at least one of wireless access systems,Institute of Electrical and Electronics Engineers (IEEE) 802, 3rdGeneration Partnership Project (3GPP), 3GPP long term evolution (3GPPLTE), LTE-advanced (LTE-A), and 3GPP2. Steps or parts that are notdescribed to clarify the technical features of the present disclosurecan be supported by those documents. Further, all terms as set forthherein can be explained by the standard documents.

Techniques described herein can be used in various wireless accesssystems such as code division multiple access (CDMA), frequency divisionmultiple access (FDMA), time division multiple access (TDMA), orthogonalfrequency division multiple access (OFDMA), single carrier-frequencydivision multiple access (SC-FDMA), etc. CDMA may be implemented as aradio technology such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be implemented as a radio technology such as globalsystem for mobile communications GSM)/general packet radio service(GPRS)/Enhanced Data Rates for GSM Evolution (EDGE). OFDMA may beimplemented as a radio technology such as IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802.20, evolved-UTRA (E-UTRA) etc. UTRA is a partof universal mobile telecommunications system (UMTS). 3GPP LTE is a partof Evolved UMTS (E-UMTS) using E-UTRA. 3GPP LTE employs OFDMA fordownlink and SC-FDMA for uplink. LTE-A is an evolution of 3GPP LTE.WiMAX can be described by the IEEE 802.16e standard (wirelessmetropolitan area network (WirelessMAN)-OFDMA Reference System) and theIEEE 802.16m standard (WirelessMAN-OFDMA Advanced System). For clarity,this application focuses on the 3GPP LTE and LTE-A systems. However, thetechnical features of the present disclosure are not limited thereto.

LTE/LTE-A Resource Structure/Channel

With reference to FIG. 1, the structure of a radio frame will bedescribed below.

In a cellular orthogonal frequency division multiplexing (OFDM) wirelesspacket communication system, uplink and/or downlink data packets aretransmitted in subframes. One subframe is defined as a predeterminedtime period including a plurality of OFDM symbols. The 3GPP LTE standardsupports a type-1 radio frame structure applicable to frequency divisionduplex (FDD) and a type-2 radio frame structure applicable to timedivision duplex (TDD).

FIG. 1(a) illustrates the type-1 radio frame structure. A downlink radioframe is divided into 10 subframes. Each subframe is further dividedinto two slots in the time domain. A unit time during which one subframeis transmitted is defined as a transmission time interval (TTI). Forexample, one subframe may be lms in duration and one slot may be 0.5 msin duration. A slot includes a plurality of OFDM symbols in the timedomain and a plurality of resource blocks (RBs) in the frequency domain.Because the 3GPP LTE system adopts OFDMA for downlink, an OFDM symbolrepresents one symbol period. An OFDM symbol may be referred to as anSC-FDMA symbol or symbol period. An RB is a resource allocation unitincluding a plurality of contiguous subcarriers in a slot.

The number of OFDM symbols in one slot may vary depending on a cyclicprefix (CP) configuration. There are two types of CPs: extended CP andnormal CP. In the case of the normal CP, one slot includes 7 OFDMsymbols. In the case of the extended CP, the length of one OFDM symbolis increased and thus the number of OFDM symbols in a slot is smallerthan in the case of the normal CP. Thus when the extended CP is used,for example, 6 OFDM symbols may be included in one slot. If channelstate gets poor, for example, during fast movement of a UE, the extendedCP may be used to further decrease inter-symbol interference (ISI).

In the case of the normal CP, one subframe includes 14 OFDM symbolsbecause one slot includes 7 OFDM symbols. The first two or three OFDMsymbols of each subframe may be allocated to a physical downlink controlchannel (PDCCH) and the other OFDM symbols may be allocated to aphysical downlink shared channel (PDSCH).

FIG. 1(b) illustrates the type-2 radio frame structure. A type-2 radioframe includes two half frames, each having 5 subframes, a downlinkpilot time slot (DwPTS), a guard period (GP), and an uplink pilot timeslot (UpPTS). Each subframe is divided into two slots. The DwPTS is usedfor initial cell search, synchronization, or channel estimation at a UE.The UpPTS is used for channel estimation and acquisition of uplinktransmission synchronization to a UE at an eNB. The GP is a periodbetween an uplink and a downlink, which eliminates uplink interferencecaused by multipath delay of a downlink signal. One subframe includestwo slots irrespective of the type of a radio frame.

The above-described radio frame structures are purely exemplary and thusit is to be noted that the number of subframes in a radio frame, thenumber of slots in a subframe, or the number of symbols in a slot mayvary.

FIG. 2 illustrates the structure of a downlink resource grid for theduration of one downlink slot. A downlink slot includes 7 OFDM symbolsin the time domain and an RB includes 12 subcarriers in the frequencydomain, which does not limit the scope and spirit of the presentdisclosure. For example, a downlink slot may include 7 OFDM symbols inthe case of the normal CP, whereas a downlink slot may include 6 OFDMsymbols in the case of the extended CP. Each element of the resourcegrid is referred to as a resource element (RE). An RB includes 12×7 REs.The number of RBs in a downlink slot, NDL depends on a downlinktransmission bandwidth. An uplink slot may have the same structure as adownlink slot.

FIG. 3 illustrates the structure of a downlink subframe. Up to threeOFDM symbols at the start of the first slot in a downlink subframe areused for a control region to which control channels are allocated andthe other OFDM symbols of the downlink subframe are used for a dataregion to which a PDSCH is allocated. Downlink control channels used inthe 3GPP LTE system include a physical control format indicator channel(PCFICH), a physical downlink control channel (PDCCH), and a physicalhybrid automatic repeat request (HARM) indicator channel (PHICH). ThePCFICH is located in the first OFDM symbol of a subframe, carryinginformation about the number of OFDM symbols used for transmission ofcontrol channels in the subframe. The PHICH delivers an HARQacknowledgment/negative acknowledgment (ACK/NACK) signal in response toan uplink transmission. Control information carried on the PDCCH iscalled downlink control information (DCI). The DCI transports uplink ordownlink scheduling information, or uplink transmission power controlcommands for UE groups. The PDCCH delivers information about resourceallocation and a transport format for a downlink shared channel(DL-SCH), resource allocation information about an uplink shared channel(UL-SCH), paging information of a paging channel (PCH), systeminformation on the DL-SCH, information about resource allocation for ahigher-layer control message such as a Random Access Responsetransmitted on the PDSCH, a set of transmission power control commandsfor individual UEs of a UE group, transmission power controlinformation, voice over Internet protocol (VoIP) activation information,etc. A plurality of PDCCHs may be transmitted in the control region. AUE may monitor a plurality of PDCCHs. A PDCCH is formed by aggregatingone or more consecutive control channel elements (CCEs). A CCE is alogical allocation unit used to provide a PDCCH at a coding rate basedon the state of a radio channel. A CCE includes a plurality of REgroups. The format of a PDCCH and the number of available bits for thePDCCH are determined according to the correlation between the number ofCCEs and a coding rate provided by the CCEs. An eNB determines the PDCCHformat according to DCI transmitted to a UE and adds a cyclic redundancycheck (CRC) to control information. The CRC is masked by an identifier(ID) known as a radio network temporary identifier (RNTI) according tothe owner or usage of the PDCCH. If the PDCCH is directed to a specificUE, its CRC may be masked by a cell-RNTI (C-RNTI) of the UE. If thePDCCH is for a paging message, the CRC of the PDCCH may be masked by apaging indicator Identifier (P-RNTI). If the PDCCH carries systeminformation, particularly, a system information block (SIB), its CRC maybe masked by a system information ID and a system information RNTI(SI-RNTI). To indicate that the PDCCH carries a random access responsein response to a random access preamble transmitted by a UE, its CRC maybe masked by a random access-RNTI (RA-RNTI).

FIG. 4 illustrates the structure of an uplink subframe. An uplinksubframe may be divided into a control region and a data region in thefrequency domain. A physical uplink control channel (PUCCH) carryinguplink control information is allocated to the control region and aphysical uplink shared channel (PUSCH) carrying user data is allocatedto the data region. To maintain the property of a single carrier, a UEdoes not transmit a PUSCH and a PUCCH simultaneously. A PUCCH for a UEis allocated to an RB pair in a subframe. The RBs of the RB pair occupydifferent subcarriers in two slots. Thus it is said that the RB pairallocated to the PUCCH is frequency-hopped over a slot boundary.

Reference Signal (RS)

In a wireless communication system, a packet is transmitted on a radiochannel. In view of the nature of the radio channel, the packet may bedistorted during the transmission. To receive the signal successfully, areceiver should compensate for the distortion of the received signalusing channel information. Generally, to enable the receiver to acquirethe channel information, a transmitter transmits a signal known to boththe transmitter and the receiver and the receiver acquires knowledge ofchannel information based on the distortion of the signal received onthe radio channel. This signal is called a pilot signal or an RS.

In the case of data transmission and reception through multipleantennas, knowledge of channel states between transmission (Tx) antennasand reception (Rx) antennas is required for successful signal reception.Accordingly, an RS should be transmitted through each Tx antenna.

RSs may be divided into downlink RSs and uplink RSs. In the current LTEsystem, the uplink RSs include:

i) Demodulation-reference signal (DM-RS) used for channel estimation forcoherent demodulation of information delivered on a PUSCH and a PUCCH;and

ii) Sounding reference signal (SRS) used for an eNB or a network tomeasure the quality of an uplink channel in a different frequency.

The downlink RSs are categorized into:

i) Cell-specific reference signal (CRS) shared among all UEs of a cell;

ii) UE-specific RS dedicated to a specific UE;

iii) DM-RS used for coherent demodulation of a PDSCH, when the PDSCH istransmitted;

iv) Channel state information-reference signal (CSI-RS) carrying CSI,when downlink DM-RSs are transmitted;

v) Multimedia broadcast single frequency network (MBSFN) RS used forcoherent demodulation of a signal transmitted in MB SFN mode; and

vi) Positioning RS used to estimate geographical position informationabout a UE.

RSs may also be divided into two types according to their purposes: RSfor channel information acquisition and RS for data demodulation. Sinceits purpose lies in that a UE acquires downlink channel information, theformer should be transmitted in a broad band and received even by a UEthat does not receive downlink data in a specific subframe. This RS isalso used in a situation like handover. The latter is an RS that an eNBtransmits along with downlink data in specific resources. A UE candemodulate the data by measuring a channel using the RS. This RS shouldbe transmitted in a data transmission area.

Modeling of MIMO System

FIG. 5 is a diagram illustrating a configuration of a wirelesscommunication system having multiple antennas.

As shown in FIG. 5(a), if the number of Tx antennas is increased toN_(T) and the number of Rx antennas is increased to N_(R), a theoreticalchannel transmission capacity is increased in proportion to the numberof antennas, unlike the case where a plurality of antennas is used inonly a transmitter or a receiver. Accordingly, it is possible to improvea transfer rate and to remarkably improve frequency efficiency. As thechannel transmission capacity is increased, the transfer rate may betheoretically increased by a product of a maximum transfer rate Ro uponutilization of a single antenna and a rate increase ratio Ri.R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For instance, in an MIMO communication system, which uses four Txantennas and four Rx antennas, a transmission rate four times higherthan that of a single antenna system can be obtained. Since thistheoretical capacity increase of the MIMO system has been proved in themiddle of 1990s, many ongoing efforts are made to various techniques tosubstantially improve a data transmission rate. In addition, thesetechniques are already adopted in part as standards for various wirelesscommunications such as 3G mobile communication, next generation wirelessLAN, and the like.

The trends for the MIMO relevant studies are explained as follows. Firstof all, many ongoing efforts are made in various aspects to develop andresearch information theory study relevant to MIMO communicationcapacity calculations and the like in various channel configurations andmultiple access environments, radio channel measurement and modelderivation study for MIMO systems, spatiotemporal signal processingtechnique study for transmission reliability enhancement andtransmission rate improvement and the like.

In order to explain a communicating method in an MIMO system in detail,mathematical modeling can be represented as follows. It is assumed thatthere are N_(T) Tx antennas and N_(R) Rx antennas.

Regarding a transmitted signal, if there are N_(T) Tx antennas, themaximum number of pieces of information that can be transmitted isN_(T). Hence, the transmission information can be represented as shownin Equation 2.s=└s ₁ ,s ₂ , . . . ,s _(N) _(T) ┘^(T)  [Equation 2]

Meanwhile, transmit powers can be set different from each other forindividual pieces of transmission information s₁, s₂, . . . , s_(N) _(T), respectively. If the transmit powers are set to P₁, P₂, . . . , P_(N)_(T) , respectively, the transmission information with adjusted transmitpowers can be represented as Equation 3.{circumflex over (s)}=[{circumflex over (s)}₁ ,ŝ ₂ , . . . ,s _(N) _(T)]^(T)=[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P _(N) _(T) s _(N) _(T)]^(T)  [Equation 3]

In addition, Ŝ can be represented as Equation 4 using diagonal matrix Pof the transmission power.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Assuming a case of configuring N_(T) transmitted signals x₁, x₂, . . . ,x_(N) _(T) , which are actually transmitted, by applying weight matrix Wto the information vector Ŝ having the adjusted transmit powers, theweight matrix W serves to appropriately distribute the transmissioninformation to each antenna according to a transport channel state. x₁,x₂, . . . , x_(N) _(T) can be expressed by using the vector X asfollows.

$\begin{matrix}{x = {\left\lbrack \begin{matrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{matrix} \right\rbrack = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\;\hat{s}} = {WPs}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

In Equation 5, W_(ij) denotes a weight between an i^(th) Tx antenna andj^(th) information. W is also called a precoding matrix.

If the N_(R) Rx antennas are present, respective received signals y₁,y₂, . . . , y_(N) _(R) of the antennas can be expressed as follows.y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Equation 6]

If channels are modeled in the MIMO wireless communication system, thechannels may be distinguished according to Tx/Rx antenna indexes. Achannel from the Tx antenna j to the Rx antenna i is denoted by h_(ij).In h_(ij), it is noted that the indexes of the Rx antennas precede theindexes of the Tx antennas in view of the order of indexes.

FIG. 5(b) is a diagram illustrating channels from the N_(T) Tx antennasto the Rx antenna i. The channels may be combined and expressed in theform of a vector and a matrix. In FIG. 5(b), the channels from the N_(T)Tx antennas to the Rx antenna i can be expressed as follows.h _(i) ^(T)=[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  [Equation 7]

Accordingly, all channels from the N_(T) Tx antennas to the N_(R) Rxantennas can be expressed as follows.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

An AWGN (Additive White Gaussian Noise) is added to the actual channelsafter a channel matrix H. The AWGN n₁, n₂, . . . , n_(N) _(R)respectively added to the N_(R) Rx antennas can be expressed as follows.n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Equation 9]

Through the above-described mathematical modeling, the received signalscan be expressed as follows.

$\begin{matrix}{y = {\left\lbrack \begin{matrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{matrix} \right\rbrack = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

Meanwhile, the number of rows and columns of the channel matrix Hindicating the channel state is determined by the number of Tx and Rxantennas. The number of rows of the channel matrix H is equal to thenumber N_(R) of Rx antennas and the number of columns thereof is equalto the number N_(T) of Tx antennas. That is, the channel matrix H is anN_(R)×N_(T) matrix.

The rank of the matrix is defined by the smaller of the number of rowsand the number of columns, which are independent from each other.Accordingly, the rank of the matrix is not greater than the number ofrows or columns. The rank rank (H) of the channel matrix H is restrictedas follows.rank(H)≤min(N _(T) ,N _(R))  [Equation 11]

Additionally, the rank of a matrix can also be defined as the number ofnon-zero Eigen values when the matrix is Eigen-value-decomposed.Similarly, the rank of a matrix can be defined as the number of non-zerosingular values when the matrix is singular-value-decomposed.Accordingly, the physical meaning of the rank of a channel matrix can bethe maximum number of channels through which different pieces ofinformation can be transmitted.

In the description of the present document, ‘rank’ for MIMO transmissionindicates the number of paths capable of sending signals independentlyon specific time and frequency resources and ‘number of layers’indicates the number of signal streams transmitted through therespective paths. Generally, since a transmitting end transmits thenumber of layers corresponding to the rank number, one rank has the samemeaning of the layer number unless mentioned specially.

Synchronization Acquisition of D2D UE

Now, a description will be given of synchronization acquisition betweenUEs in D2D communication based on the foregoing description in thecontext of the legacy LTE/LTE-A system. In an OFDM system, iftime/frequency synchronization is not acquired, the resulting inter-cellinterference (ICI) may make it impossible to multiplex different UEs inan OFDM signal. If each individual D2D UE acquires synchronization bytransmitting and receiving a synchronization signal directly, this isinefficient. In a distributed node system such as a D2D communicationsystem, therefore, a specific node may transmit a representativesynchronization signal and the other UEs may acquire synchronizationusing the representative synchronization signal. In other words, somenodes (which may be an eNB, a UE, and a synchronization reference node(SRN, also referred to as a synchronization source)) may transmit a D2Dsynchronization signal (D2DSS) and the remaining UEs may transmit andreceive signals in synchronization with the D2DSS.

D2DSSs may include a primary D2DSS (PD2DSS) or a primary sidelinksynchronization signal (PSSS) and a secondary D2DSS (SD2DSS) or asecondary sidelink synchronization signal (SSSS). The PD2DSS may beconfigured to have a similar/modified/repeated structure of a Zadoff-chusequence of a predetermined length or a primary synchronization signal(PSS). Unlike a DL PSS, the PD2DSS may use a different Zadoff-chu rootindex (e.g., 26, 37). And, the SD2DSS may be configured to have asimilar/modified/repeated structure of an M-sequence or a secondarysynchronization signal (SSS). If UEs synchronize their timing with aneNB, the eNB serves as an SRN and the D2DSS is a PSS/SSS. Unlike PSS/SSSof DL, the PD2DSS/SD2DSS follows UL subcarrier mapping scheme. FIG. 6shows a subframe in which a D2D synchronization signal is transmitted. Aphysical D2D synchronization channel (PD2DSCH) may be a (broadcast)channel carrying basic (system) information that a UE should firstobtain before D2D signal transmission and reception (e.g., D2DSS-relatedinformation, a duplex mode (DM), a TDD UL/DL configuration, a resourcepool-related information, the type of an application related to theD2DSS, etc.). The PD2DSCH may be transmitted in the same subframe as theD2DSS or in a subframe subsequent to the frame carrying the D2DSS. ADMRS can be used to demodulate the PD2DSCH.

The SRN may be a node that transmits a D2DSS and a PD2DSCH. The D2DSSmay be a specific sequence and the PD2DSCH may be a sequencerepresenting specific information or a codeword produced bypredetermined channel coding. The SRN may be an eNB or a specific D2DUE. In the case of partial network coverage or out of network coverage,the SRN may be a UE.

In a situation illustrated in FIG. 7, a D2DSS may be relayed for D2Dcommunication with an out-of-coverage UE. The D2DSS may be relayed overmultiple hops. The following description is given with the appreciationthat relay of an SS covers transmission of a D2DSS in a separate formataccording to a SS reception time as well as direct amplify-and-forward(AF)-relay of an SS transmitted by an eNB. As the D2DSS is relayed, anin-coverage UE may communicate directly with an out-of-coverage UE.

D2D Resource Pool

FIG. 8 shows an example of a first UE (UE1), a second UE (UE2) and aresource pool used by UE1 and UE2 performing D2D communication. In FIG.8(a), a UE corresponds to a terminal or such a network device as an eNBtransmitting and receiving a signal according to a D2D communicationscheme. A UE selects a resource unit corresponding to a specificresource from a resource pool corresponding to a set of resources andthe UE transmits a D2D signal using the selected resource unit. UE2corresponding to a receiving UE receives a configuration of a resourcepool in which UE1 is able to transmit a signal and detects a signal ofUE1 in the resource pool. In this case, if UE1 is located at the insideof coverage of an eNB, the eNB can inform UE1 of the resource pool. IfUE1 is located at the outside of coverage of the eNB, the resource poolcan be informed by a different UE or can be determined by apredetermined resource. In general, a resource pool includes a pluralityof resource units. A UE selects one or more resource units from among aplurality of the resource units and may be able to use the selectedresource unit(s) for D2D signal transmission. FIG. 8(b) shows an exampleof configuring a resource unit. Referring to FIG. 8(b), the entirefrequency resources are divided into the N_(F) number of resource unitsand the entire time resources are divided into the N_(T) number ofresource units. In particular, it is able to define N_(F)*N_(T) numberof resource units in total. In particular, a resource pool can berepeated with a period of N_(T) subframes. Specifically, as shown inFIG. 8, one resource unit may periodically and repeatedly appear. Or, anindex of a physical resource unit to which a logical resource unit ismapped may change with a predetermined pattern according to time toobtain a diversity gain in time domain and/or frequency domain. In thisresource unit structure, a resource pool may correspond to a set ofresource units capable of being used by a UE intending to transmit a D2Dsignal.

A resource pool can be classified into various types. First of all, theresource pool can be classified according to contents of a D2D signaltransmitted via each resource pool. For example, the contents of the D2Dsignal can be classified into various signals and a separate resourcepool can be configured according to each of the contents. The contentsof the D2D signal may include a scheduling assignment (SA or physicalsidelink control channel (PSCCH)), a D2D data channel, and a discoverychannel. The SA may correspond to a signal including information on aresource position of a D2D data channel, information on a modulation andcoding scheme (MC S) necessary for modulating and demodulating a datachannel, information on a MIMO transmission scheme, information on atiming advance (TA), and the like. The SA signal can be transmitted onan identical resource unit in a manner of being multiplexed with D2Ddata. In this case, an SA resource pool may correspond to a pool ofresources that an SA and D2D data are transmitted in a manner of beingmultiplexed. The SA signal can also be referred to as a D2D controlchannel or a physical sidelink control channel (PSCCH). The D2D datachannel (or, physical sidelink shared channel (PSSCH)) corresponds to aresource pool used by a transmitting UE to transmit user data. If an SAand a D2D data are transmitted in a manner of being multiplexed in anidentical resource unit, D2D data channel except SA information can betransmitted only in a resource pool for the D2D data channel. In otherword, REs, which are used to transmit SA information in a specificresource unit of an SA resource pool, can also be used for transmittingD2D data in a D2D data channel resource pool. The discovery channel maycorrespond to a resource pool for a message that enables a neighboringUE to discover transmitting UE transmitting information such as ID ofthe UE, and the like.

Although contents of D2D signal are identical to each other, it may usea different resource pool according to a transmission/receptionattribute of the D2D signal. For example, in case of the same D2D datachannel or the same discovery message, the D2D data channel or thediscovery signal can be classified into a different resource poolaccording to a transmission timing determination scheme (e.g., whether aD2D signal is transmitted at the time of receiving a synchronizationreference signal or the timing to which a prescribed timing advance isadded) of a D2D signal, a resource allocation scheme (e.g., whether atransmission resource of an individual signal is designated by an eNB oran individual transmitting UE selects an individual signal transmissionresource from a pool), a signal format (e.g., number of symbols occupiedby a D2D signal in a subframe, number of subframes used for transmittinga D2D signal), signal strength from an eNB, strength of transmit powerof a D2D UE, and the like. For clarity, a method for an eNB to directlydesignate a transmission resource of a D2D transmitting UE is referredto as a mode 1 (mode 3 in case of V2X). If a transmission resourceregion is configured in advance or an eNB designates the transmissionresource region and a UE directly selects a transmission resource fromthe transmission resource region, it is referred to as a mode 2 (mode 4in case of V2X). In case of performing D2D discovery, if an eNB directlyindicates a resource, it is referred to as a type 2. If a UE directlyselects a transmission resource from a predetermined resource region ora resource region indicated by the eNB, it is referred to as type 1.

SA Transmission/Reception

A mode-1 UE may transmit an SA (D2D control signal, or sidelink controlinformation (SCI)) in resources configured by an eNB. For a mode-2 UE,the eNB configures resources for D2D transmission. The mode-2 UE mayselect time-frequency resources from the configured resources andtransmit an SA in the selected time-frequency resources.

An SA period may be defined as illustrated in FIG. 9. Referring to FIG.9, a first SA period may start in a subframe spaced from a specificsystem frame by a predetermined offset, SAOffsetlndicator indicated byhigher-layer signaling. Each SA period may include an SA resource pooland a subframe pool for D2D data transmission. The SA resource pool mayinclude the first subframe of the SA period to the last of subframesindicated as carrying an SA in a subframe bitmap, saSubframeBitmap. Theresource pool for D2D data transmission may include subframes used foractual data transmission through application of a time-resource patternfor transmission (T-RPT) or a time-resource pattern (TRP) in mode 1. Asillustrated, if the number of subframes included in an SA period exceptfor an SA resource pool is larger than the number of T-RPT bits, theT-RPT may be applied repeatedly, and the last applied T-RPT may betruncated to be as long as the number of remaining subframes. Atransmitting UE performs transmission at positions corresponding to isset in a T-RPT bitmap in an indicated T-RPT, and transmits one mediumaccess control layer protocol data unit (MAC PDU) four times.

In V2V communication, a cooperative awareness message (CAM) of aperiodic message type, a decentralized environmental notificationmessage (DENM) of an event triggered message type, and so on may betransmitted. The CAM may deliver basic vehicle information includingdynamic state information about a vehicle, such as a direction and aspeed, static data of the vehicle, such as dimensions, an ambientillumination state, details of a path, and so on. The CAM may be 50bytes to 300 bytes in length. The CAM is broadcast, and its latencyshould be shorter than 100 ms. The DENM may be generated, uponoccurrence of an unexpected incident such as breakdown or an accident ofa vehicle. The DENM may be shorter than 3000 bytes, and received by allvehicles within a transmission range. The DENM may have a higherpriority than the CAM. When it is said that a message has a higherpriority, this may mean that from the perspective of one UE, in the caseof simultaneous transmission of messages, the higher-priority message istransmitted above all things, or earlier in time than any other of theplurality of messages. From the perspective of multiple UEs, a messagehaving a higher priority may be subjected to less interference than amessage having a lower priority, to thereby have a reduced receptionerror probability. Regarding the CAM, the CAM may have a larger messagesize when it includes security overhead than when it does not.

Embodiment

A D2D/V2X UE may acquire synchronization from at least one of a globalnavigation satellite system (GNSS) or an eNB, and transmit a D2D signalbased on the acquired synchronization. When the UE is (a) located withina time division duplex (TDD) cell and (b) an in-coverage UE, and (c) theeNB configures a system frame number (SFN) boundary based on a GNSStiming, the UE may apply a predetermined offset to transmission of a D2Dsignal, irrespective of whether the UE uses the GNSS as asynchronization reference. In the case where an in-coverage UE is withinthe coverage of a TDD cell, a predetermined offset is introducedirrespective of whether the GNSS is used as a synchronization referenceor timing reference. In this case, a timing difference may occur betweenan in-coverage UE that configures the GNSS as a synchronizationreference and an out-coverage UE that configures the GNSS as asynchronization reference. Thus, the in-coverage UE may transmitinformation indicating whether a timing offset is applied on a physicalsidelink broadcast channel (PSBCH). Upon receipt of the PSBCH, theout-coverage UE may apply the predetermined timing offset totransmission of a D2D signal. Even though the UE configures the GNSS asa synchronization reference, when the in-coverage UE signals an offseton the PSBCH, the UE transmits a UL/SL signal, using the offset.

This method is advantageous in that in the case where an eNB configuresa SFN boundary based on a GNSS timing, when D2D frame numbers (DFNs) arealigned with SFNs, a switching time for UL/SL-DL switching may besecured.

FIG. 10 illustrates such an example as described above. Referring toFIG. 10, it is assumed that a first in-coverage UE (UE1) and a secondout-coverage UE (UE2) are synchronized with the GNSS, and a thirdin-coverage UE (UE3) is synchronized with an eNB. In the exampleillustrated in FIG. 10, if UE1 is a) located within a TDD cell and b) anin-coverage UE, and c) the eNB configures an SFN boundary based on aGNSS timing, UE1 may apply a predetermined offset to transmission of aD2D signal irrespective of whether the UE uses the GNSS as asynchronization reference. This offset is transmitted to UE2 on a PSBCH.Upon receipt of the offset, UE2 being an out-coverage UE applies thepredetermined offset to transmission of a D2D signal. Therefore, asillustrated in FIG. 10(a), all of UE1, UE2, and UE3 have the sametiming, and transmit signals with the predetermined same offset.

If a UE is a) located within a TDD cell and b) an in-coverage UE, c) aneNB does not configure an SFN boundary based on a GNSS timing, and d)the UE uses the GNSS as a synchronization reference, the UE may apply aD2D frame number (DFN) offset to transmission of a D2D signal. That is,if the UE configures the GNSS as a synchronization reference, the UEdoes not apply a predetermined offset. Herein, an in-coverage UE thathas configured the GNSS as a synchronization reference among in-coverageUEs may not be aligned with a cellular UE, in terms of timing. Thus, anetwork may configure a DFN offset such that the DFN offset correspondsto a position obtained by applying the predetermined offset (for Tx/Rxswitching) to an SFN. That is, the DFN offset may be obtained byapplying the predetermined offset to the difference between an SFN and aDFN. This DFN offset may be set to be equal to a DFN pre-configurationoffset for an out-coverage UE. This is exemplified in FIG. 10(b). UE1and UE2 have GNSS-based DFNs, and UE3 has eNB-based SFNs. Since UE1 andUE2 transmit signals by applying a DFN offset resulting from applying apredetermined offset to the difference between an SFN and a DFN to thesignals, UE1 and UE2 may have the same timing as that of UE3 whichtransmits a signal by applying the predetermined offset based on an SFN.That is, this method is advantageous in that among UEs that haveconfigured the GNSS as a synchronization reference, an in-coverage UEand an out-coverage UE may have the same timing.

This method may enable use of the same timing between UEs which haveconfigured the GNSS as a synchronization reference, and application ofan offset between a DL subframe and a UL/SL subframe of a Uu link,thereby ensuring Tx/Rx switching. In this method, a predetermined offsetis applied between an SFN and a DFN. Then, a special subframe of an SFNand a UL subframe of a DFN may partially overlap with each other.Herein, it may be determined that a time difference within the foregoingpredetermined offset does not lead to overlap between subframes. Thatis, even though a specific subframe partially overlaps with a subframeof a UE located within a TDD cell, the UE may not exclude the specificsubframe from D2D subframes. In other words, even though a UL subframepartially overlaps with a special subframe of Uu, the UE exceptionallyneither drops a packet nor excludes the subframe from V2V subframes.

Even when a UE performs sidelink carrier aggregation (CA), if each celluses a different duplex mode, the problem of different subframeboundaries may occur. For example, If TDD is used in component carrier 1(CC1) and FDD is used in CC2, a 624Ts offset may be applied to the TDDCC, not to the FDD CC in CA. Then, the 624Ts offset may also always beapplied to the FDD cell, or a DFN offset may be configured separatelyfor each cell so that the subframe boundaries of the TDD CC and the FDDCC may be aligned with each other at the DFN offsets. If an offset isapplied to the FDD cell, some UL subframes may overlap with each other.In this case, it may be regulated that a UL subframe previous to asubframe configured as a sidelink resource pool should be dropped.

The foregoing descriptions are applicable to UL or DL, not limited todirection communication between UEs. Herein, an eNB or a relay node mayuse the proposed methods.

Since examples of the above proposed methods may be included as one ofmethods of implementing the present disclosure, it is apparent that theexamples may be regarded as proposed methods. Further, the foregoingproposed methods may be implemented independently, or some of themethods may be implemented in combination (or merged). Further, it maybe regulated that information indicating whether the proposed methodsare applied (or information about the rules of the proposed methods) isindicated to a UE by a pre-defined signal (or a physical-layer orhigher-layer signal) by an eNB.

Apparatus Configurations According to Embodiment of the PresentDisclosure

FIG. 11 is a block diagram of a transmission point and a UE according toan embodiment of the present disclosure.

Referring to FIG. 11, a transmission point 10 according to the presentdisclosure may include a receiver 11, a transmitter 12, a processor 13,a memory 14, and a plurality of antennas 15. Use of the plurality ofantennas 15 means that the transmission point 10 supports MIMOtransmission and reception. The receiver 11 may receive various ULsignals, data, and information from a UE. The transmitter 12 maytransmit various DL signals, data, and information to a UE. Theprocessor 13 may provide overall control to the transmission point 10.The processor 13 of the transmission point 10 according to an embodimentof the present disclosure may process requirements for each of theforegoing embodiments.

Besides, the processor 13 of the transmission point 10 may function tocompute and process information received by the transmission point 10and information to be transmitted to the outside. The memory 14 maystore the computed and processed information for a predetermined time,and may be replaced by a component such as a buffer (not shown).

With continued reference to FIG. 11, a UE 20 according to the presentdisclosure may include a receiver 21, a transmitter 22, a processor 23,a memory 24, and a plurality of antennas 15. Use of the plurality ofantennas 25 means that the UE 20 supports MIMO transmission andreception. The receiver 21 may receive various DL signals, data, andinformation from an eNB. The transmitter 22 may transmit various ULsignals, data, and information to an eNB. The processor 23 may provideoverall control to the UE 20.

The processor 23 of the UE 20 according to an embodiment of the presentdisclosure may process requirements for each of the foregoingembodiments. Specifically, the processor acquires synchronization fromat least one of a GNSS or an eNB, and transmits a D2D signal based onthe acquired synchronization through the transmitter. When the UE is a)located within a TDD cell and b) an in-coverage UE, and c) the eNBconfigures an SFN boundary based on a GNSS timing, the processor mayapply a predetermined offset to transmission of a D2D signal,irrespective of whether the UE uses the GNSS as a synchronizationreference.

The processor 23 of the UE 20 may also perform a function ofcomputationally processing information received by the UE 20 andinformation to be transmitted to the outside, and the memory 24 maystore the computationally processed information and the like for apredetermined time and may be replaced by a component such as a buffer(not shown).

The specific configuration of the transmission point apparatus and theUE may be implemented such that the details described in the variousembodiments of the present invention may be applied independently orimplemented such that two or more of the embodiments are applied at thesame time. For clarity, redundant description is omitted.

In the example of FIG. 11, the description of the transmission pointapparatus 10 may also be applied to a relay device as a downlinktransmission entity or an uplink reception entity, and the descriptionof the UE 20 may also be applied to a relay device as a downlinkreception entity or an uplink transmission entity.

The embodiments of the present invention may be implemented throughvarious means, for example, hardware, firmware, software, or acombination thereof.

In a hardware configuration, the embodiments of the present disclosuremay be achieved by one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, microcontrollers,microprocessors, etc.

In a firmware or software configuration, a method according toembodiments of the present disclosure may be implemented in the form ofa module, a procedure, a function, etc. Software code may be stored in amemory unit and executed by a processor. The memory unit is located atthe interior or exterior of the processor and may transmit and receivedata to and from the processor via various known means.

As described before, a detailed description has been given of preferredembodiments of the present disclosure so that those skilled in the artmay implement and perform the present disclosure. While reference hasbeen made above to the preferred embodiments of the present disclosure,those skilled in the art will understand that various modifications andalterations may be made to the present disclosure within the scope ofthe present disclosure. For example, those skilled in the art may usethe components described in the foregoing embodiments in combination.The above embodiments are therefore to be construed in all aspects asillustrative and not restrictive. The scope of the disclosure should bedetermined by the appended claims and their legal equivalents, not bythe above description, and all changes coming within the meaning andequivalency range of the appended claims are intended to be embracedtherein.

Those skilled in the art will appreciate that the present disclosure maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent disclosure. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of thedisclosure should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. It is obvious to those skilled in the art thatclaims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentdisclosure or included as a new claim by a subsequent amendment afterthe application is filed.

INDUSTRIAL APPLICABILITY

The above-described embodiments of the present disclosure are applicableto various mobile communication systems.

The invention claimed is:
 1. A method of acquiring synchronization andtransmitting a device-to-device (D2D) signal by a user equipment (UE) ina wireless communication system, the method comprising: acquiringsynchronization from at least one of a global navigation satellitesystem (GNSS) or an evolved Node B (eNB); and transmitting a D2D signalbased on the acquired synchronization, wherein based on a) the UE beinglocated within a time division duplex (TDD) cell, b) the UE being anin-coverage UE, and c) the eNB configuring a system frame number (SFN)boundary in a time domain based on a GNSS timing, the UE applies a firstoffset to transmission of a D2D signal irrespective of whether the UEuses the GNSS as a synchronization reference, wherein the first offsetis related to a transmission/reception (Tx/Rx) switching time of the UE,wherein the UE applies a second offset to transmission of a D2D signal,based on a) the UE being located within the TDD cell, b) the UE being anin-coverage UE, and c) the UE using the GNSS as a synchronizationreference, and wherein the second offset is a sum of the first offsetand a timing difference between an SFN and a D2D frame number (DFN). 2.The method according to claim 1, wherein the D2D signal includesinformation indicating whether the first offset is applied in a physicalsidelink broadcast channel (PSBCH).
 3. The method according to claim 2,wherein based on receipt of the PSBCH, an out-coverage UE applies thefirst offset to transmission of a D2D signal.
 4. The method according toclaim 1, wherein the second offset is equal to an offset preconfiguredfor an out-coverage UE.
 5. The method according to claim 1, wherein aspecial subframe which partially overlaps with a subframe of a UE withinthe TDD cell is included in a D2D subframe.
 6. A user equipment (UE)configured to acquire synchronization and transmit a device-to-device(D2D) signal in a wireless communication system, the UE comprising: atransmitter and a receiver; and a processor, wherein the processor isconfigured to acquire synchronization from at least one of a globalnavigation satellite system (GNSS) or an evolved Node B (eNB), and totransmit a D2D signal based on the acquired synchronization through thetransmitter, and wherein based on a) the UE being located within a timedivision duplex (TDD) cell, b) the UE being an in-coverage UE, and c)the eNB configuring a system frame number (SFN) boundary in a timedomain based on a GNSS timing, the processor is configured to apply afirst offset to transmission of a D2D signal irrespective of whether theUE uses the GNSS as a synchronization reference, and wherein the firstoffset is related to a transmission/reception (Tx/Rx) switching time ofthe UE, wherein the UE applies a second offset to transmission of a D2Dsignal, based on a) the UE being located within the TDD cell, b) the UEbeing an in-coverage UE, and c) the UE using the GNSS as asynchronization reference, and wherein the second offset is a sum of thefirst offset and a timing difference between an SFN and a D2D framenumber (DFN).
 7. The UE according to claim 6, wherein the D2D signalincludes information indicating whether the first offset is applied in aphysical sidelink broadcast channel (PSBCH).
 8. The UE according toclaim 7, wherein based on receipt of the PSBCH, an out-coverage UEapplies the first offset to transmission of a D2D signal.
 9. The UEaccording to claim 6, wherein the second offset is equal to an offsetpreconfigured for an out-coverage UE.
 10. The UE according to claim 6,wherein a special subframe partially overlapping with a subframe of a UEwithin the TDD cell is included in a D2D subframe.